Radio alarm system



Jan. 16, 1945. L R sHlcK l A RADIO ALARM SYSTEM Filed'Auz. 2e, 1942 2 sheets-spew 1 "Ill- @NGE Jan. 16, 1945. J.; R. scHlcK 2,367,378

RADIO ALARM SYSTEM Filed Aug. 26, 1942 2 Sheets-Sheet 2 Y 3 ventor John, R. schm@ Gttorneg Patented Jan. `16, 1945 RADIO ALARM SYSTEM John R. '.Schck, Santa Ana, Calif., assigner'y to Radio Corporation of America, a corporation of f Delaware lAppuemnm August 26, 1942, serial No. 456,141`

7 claims.` f (c1. 25o-20) My present invention relates to a radio-blackout alarm system, and more particularly yto a radio receiver employing'anaudible alarm circuit.

The usual arrangement employedfor obtain- .ing notice of a radio black-out, or shutdown, is to have a broadcast receiver running rcontinuously on one ofthe key stations monitored for this purpose. In other words, each broadcast transmitter in a given area provides at its station a monitor broadcast receiver which constantly reproduces the program of the keystation of that area. The disadvantages of this arrangement are .that it is necessary to have thebroadcast monitor receiver operating at an amplitude that" causes constant distraction, and. is detrimentall tothe operation of the business where the receiver is located. Again, it is sometimes desirable, or necessary, that the monitor engineer be listening to other equipment.

If the monitor operator is moving from room to y room, the outputl amplitude of the receiver must be maintained very high, or it .must be continually adjusted. Itis evident that -it is desirableto have a radio alarm system that will not make it necessary for the operator continually to monitor controlled from vide an audio frequency feedback system in a radio receiver, and the. feedback being effectively A the receiver demodulator so that a loud alarm is heard from the reproducer in response to disappearancev of ja predetermined station carrier.

Still other objects of my invention are to improve the simplicity and efficiency of radio blackout alarm systems, and more especially to provide systems which are durable, reliable and economical to manufacture and assemble. A

The novel features which I believe to be 'characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the-following description taken in connection with'the drawings, in vwhich I Vhave indicated diagrammatically several circuit organifzations whereby my invention may be vrcarried into effect.

the modulation of the key station. It is desirable to have the attention of the operator called by an alarm when the key station carrier isy either removed or applied. Again, it is desirable that whatever arrangement is used, it be simple, mex-,

pensive and require no adjustments.

In the drawings: Fig. .-1 shows a radio receiver embodyi'ngtheinvention.

Fig. 2 shows a modified form of audio frequency feedback circuit,

Fig. 3 illustratesy an lectronic form of feedback control relay,

It may, therefore, be stated that it is one of the main objects of my present invention to provide a radio alarm arrangement which. is applicable to a conventional radio. broadcast receiver, and .is

able to provide an audible alarm which will indicate either the removal, or the' application, of the carrier of the key station to which themonitor receiver is tuned. A f

Another important object of my invention is to provide a radio alarm system wherein areceiver is tuned to a key station, andthere being rincluded means fcroperating the receiver at any amplitude including sub-audible; a loud audible signal emanating from the receiver reproducer in response tothe key station carrier `being re-r moved. f

Another object of my invention is to provide a radio receiver which employs either a mechanical orvacuum tube relay, adapted to be actuated by Fig. 4 shows a modication in the mode of cony trol ofthe relay, the control being from the de- .modulaton Fig. 5 shows a modification of the' arrangementof Fig.4.

Referring nowto the accompanying drawings, wherein like reference characters in the different figures denote similar circuit elements, the receiver of Fig. 1 isfassumed -to be a superheteroa controlled amplifier stage,v to introduce feedi back into one or more audio frequency amplifier "stages thereby to produce a loud audio frequency Lhowl to simulate a sireneiect.

invention is` to pro- Still another object of the dynel broadcast receiver which operates in the usual550 to 1700 kilocycles (kc.) band.` Those skilled in the radio art wellknow the manner of constructing such a receiver. It is to be understood, however, thatmy invention is independent of the receiver operating range, the type of receiver,v orv the nature of the carrier modulation. For example, the invention may be applied to a frequency modulated (FM) carrier wave receiver,

as well. as to an amplitude modulated carrier wave receiver.

In the schematic system of Fig. 1, the receiver mayA comprise the usual signal wave collector, such as a grounded antenna, which feeds the sig- .nals to one. or more tuned radio frequency ampliers., The selector circuits of. the amplifiers, as

well"a,s 'the selective input circuit of the converter, willbe kept tuned to the carrier Vfrequency automatic volume control (AVC) bias.

of the key station of a given area. One transmitting station of a given broadcast area of stations is termed a key station. All other broadcasters. as well as point to point stations, in that area will have receivers tuned to the key station carrier frequency. Of course, the homes of all persons in that area may, also, equip: themselves with the present "system so that they'rnay concurrentlybe-aware ofi-the black-out-signal. For example, air-raid wardens may use the system.

The converter, of any desired type, acts to reduce the carrier frequency to the intermediate frequency (I. F.).- The I. F. energy is amplified to one or more amplifier stages. The last I. F. amplifier tube is designatedas/I. `Its linputelec.` trodes G and F are coupled to the secondary winding of an I. F. input 4transformer 2. The I. F. output transformer 3 couples the output electrodes P and F of tube I to the diode-.demodulator electrodes 5 and 5.

The diode Aisfincluded `in a common tube venvelope with a'triode Asection acting 'asthe-first audio 'frequencya-rnplifier.f f' This is true for-some receivers only, as the Iaudio' frequency 'amplier may be in a separate tube. Tube 4 maybe ofthe I diode-triode type. The .common cathode ,5I 'ls grounded, While the .diode anode 5 is connected to the high potential side of the secondary circuit of transformer '3. vrOf course, the usual 7bypassed biasing resistor may be used in the cathode lead. 'I'he lowside ofthe diode input circuit is connectedto ground througlia path comprising resistor 6 and a .predetermined Aportion of re- .sistor 1. ,The 'arm `V of resistor'is slidable along resistor 1 to provide a .volume control; acon- Astant impedance .to the=grid 4 of the first audio vfrequencyampliiier to ground is provided by this arrangement. The upper end `of resistor is bypassed togroundforI. F. currents by .condenser 6'. 'I'he voltage drop developed due to rectified current flowing through resistor 6 Ais used as Theactual load on the demodulator is a variable, and `comprises resistor plus that portion of resistor I between the movable arm V and ground.

As shown, the AVC lead 8 is arranged `to be connected from the upper end of resistor ,Btothe signal grids ofthe various controlledstages. The AVC `lead includes filter resistor 9-to filter Voutfall audio pulsating voltage components. The lowfpotential side of the secondary circuit of transformer 2 is connected to the AVC lead. In the absence of received carrier energy there Y.is no rectified current flowing through resistor 6 to ground. Hence, the AVC bias is zero, and the `controlled stages have normal, 'maximum lgain. When carrier'energyis received, there is `developed AVCbias which is applied in a-negative,-or gain-reducing, sense to thefcontrolled amplifiers.

Accordingly. the plate-current of each vcontrolled tube is substantially reduced.

I make use ofthis changein plate current of the I.`F. amplifier tube kI by inserting `the winding of an electromagnet `IIl'in the circuito! plate -P of ampliiier tube I. A pivoteii, metallic arma- 'ture II, which is normally biasedby spring I I',

,is being received, and,'hence, lthe plate current flow through relay I0 will be insuiiicient'to hold vriection'to point I3 the receiver is in carrier oil position. When the switch I6 is adjusted to be .connected to point I2, then the receiver is in carrier on position. The switch arm I6 is connected by lead I8 to the control grid 4' of the vtriode section of tube 4. The control grid 4' may zbeconnected to the cathode 5 by a grid leak resistor 6". The plate I8 of the triode section `of tube 4 is coupled, as by resistance-capacity coupling, Vto a following audio frequency amplier 1I 9 of conventional construction. Tube I9 comprises the `catho-de I9', grid I9 and plate I4'. 'The output audio frequency transformer 2li 'may couple vthe plate circuit of audio frequency 'tube I 9 to any desired type of reproducer, for eX- ample a loudspeaker. The audio frequency amplifier network is purely conventional, and need not be described in any further detail.

To provide a siren effect, there is connected an 'audio frequency feedback path from the circuit ofplate I4 of audio frequency tube I9 to the piv- The audio frequencyv oted end of armature II. feedback path includes the condenser Ia'which has `a, low impedancel to audio frequency currents.

'The audio frequency input grid 4' of tube 4 is 'connected tothe ungrounded end of resistor l by condenser 1. In this way the audio frequency `voltage component of the rectified I. F. current is vapplied to the control grid of tube 4.

yIn explaining the operation of the present invention, it is first pointed out that the spring tension 4on armature II is so adjusted that the lrelayfis closed when no carrier is received. When signal carrier energy is applied to the receiver,

'theAVC action causes the plate current flowing through the relay coil to decrease. The relay I0 must be lsensitive enough to operate on this change'in current. The front and back contacts I3 and I2 of the relay are connected to the reversing switch I6, whose adjustable arm is connectedto tle ,input grid 4 of the first audio frequency amplifier.l

With the switch I6 thrown to the fi'oni-l contact I3, a signal may be tuned in on the receiver andthe audio frequency level in the speaker output adjusted to any desirable level, or turned down to sub-audible. The feedback circuit through the relay contacts is not complete due ,to the contacts being open. However, when the signal carrier is removed, the armature II will ycontact I3, and audio frequency feedback will be vintroduced through the condenser I4 from the last plateI4 tothe amplifier grid 4. The audio .frequency system will'oscillate, and cause a loud .Xed amount of resistance is present in the audio 'frequency grid circuit o-f the first audio frequency amplifier grid. In this way the vsetting of the volume control along .resistor I has negligible effect on the audio frequency feedback. A bypass condenser I1 shunts the I. F. current around the relay I0. The lead to the flrstaudio grid may be shielded, as sho-wn by the dotted line on either the switch II'will be yadjusted to the contact I3 position. vIt is only when a'black-outalarm is indicated by the loudspeaker, which is represented by the howling siren/effect, that the switchl6 is adjusted to thecontact I2 position. After Vsuch adjustment the reappearance of the howling indicates that'the carrier has come on again. This is representative of vanl all clear signal. y

'It is not essential to thel present invention that the feedback path bey made to the first audio frequency amplifier. Inv'Fig.- 2 there is 4shown a modificationy wherein the audio frequency feedback is accomplished solely in the last audio frequency amplifier stage. It'will beunderstood that the receiving system'in this case is similar to that shown in Fig. l, with the exception of the changes now to be described. The armature I I is connected by lead'2I and iron core coil 22 to the grid I9" of audio frequency amplifier I9.` 'I'he switch arm of switch I6 is connected by lead I8' to an intermediate tap'on coil v22. The cathode I9" of tube I9' is connected to the, intermediate tap on coil 22 vthrough a bypassed resistor 23. In this way the audio frequency amplifier I9 is caused to oscillate at an audio frequency. `The contacts I 2` and I3 yare so connected that a portion of coil 22 is shorted out when oscillation is not desired thereby converting the tube to a sinr-` ple impedance-coupledv audio frequency amplifier...

If the receiver was tuned to,- and receiving, a carrier,the switch arm I6 would be thrown to the lower (I2) position. In this case the lbottom half.

of the impedance 22 would be shorted out. The impedance 22 is for audio frequencies,`and has an iron core. When the lcarrier is removedthe I. F. plate current flowing vthrough the relay coil I0 will increase. This will cause the relay armature Ato move to contact I3. Hence, the lower half of impedance 22 wil1 be introduced bringing the cathode of tube I9 above ground so far as audio frequencies are concerned, and oscillations will be provided. The finalr tube I9 is a pentode, or beam, tubewhich has a screen grid `22 bypassed to ground for audio frequencies. Thus, considering 'the screen 22' of tube I9 as the'plate of the oscillating circuit, there is provided a simple Hartleyloscillator. The specific methods employed to rst audio frequency tube. The variable resistor 26 controls the intensity of audio frequency feedback.

The electronic relay operates on the principle that the small voltage applied to the grid of relay tubel 25 will cause large changes lin the plate irnpedance of that tube, This variation in plate impedance is sufficient to start or stop oscillation, as it is placed in series with the feedback circuit. In other words, when carrier is being received there isa large negative bias on the grid G' of tube 25. This means that there is a large plate impedance in series in the audio frequency feedback circuit. Hence, there will be no audio frequency howling. On the other hand, when the carrier disappears the bias on the grid of tube 25 is a minimum, and the plate impedance thereof is very small thereby permitting howling to occur.

Iii-Fig. 4 there is shown a modification of the arrangement shown in Fig. 3 wherein a different form of electronic relay is employed. The relay tube 3 I. has its cathode connected to ground, while the plate 32 is connected by lead 33 to the plate I 8 of the first audio frequency amplifier. It will tbe noted that the plate oftube 4, the plate 32 '4' of tube 4 by means of the adjustable poten tioineter tapv 40 which is slidable along a portion ofthe diode load. The AVC bias developed across load 4I-42, which load is bypassed to ground for I. F. currents, is applied to the first grid 43 of relay. tube 3|. This embodiment involves less expensive parts, and has certain ad'- vantages over the triode relay shown in Fig. 3. For example, there exists better isolation and freedom from stray capacities throughthe relay tube so far as the feedback circuit is concerned.

. In this circuit the audio frequency feedback path produce the audio frequency oscillationsV is not f limited to those shown; the invention broadly contemplates a 'carrier-#controlled means for starting and stopping audio oscillations.

The electromagnetic relay Illis merelyl illustrative. In place-of sucha relay there may beem` ployed an electronic'relay device. In Fig. 3 there p is shown a modification of the invention wherein a vacuum tube 25, ywhich is shown as of the triode I type, has its plate 25 connected to the out-put circuit of audio frequency amplifier I9 through a variable resistor 26. The cathode F of tube .25 is connected to ground through a choke coil 21. This coil 21 prevents the grid 4 of the first audio frequency amplifier from being grounded for audio frequency currentsl through condenser 30. The control grid G` of tube 25) is connected to the AVC line 8 through a filter resistor 28, the grid end of resistor 28 being connected to ground by condenser 29. biased by 'the voltage developed across the rsecond The grid G' of relay tube 25 is.-y

detectorload 'resistor 6. The cathode and plate of the relay tube 25 are connected to the input is from the output of amplifier I9 to its grid cir'- cuit through tube 3l. The AVC bias, which is a function of the carrier magnitude, varies the bias of grid 43, and, therefore, determines the plate impedance in the vaudio frequency feed'- back path. When the carrier disappears the plate impedance of tube `I-II is a minimum, and audio frequency howling is produced. f With the proper relay tube 3I and sufficient voltage from the demodulator, the latter depending uponthe gain at the front portion of the receiver, the relay tube grid 43 will be biased sufficiently negative to prevent the feedback circuit from causing oscillations.

In Fig. 5 I have shown a further modifica- 'tion of the invention. In this modification the Switch I6 has its adjustable arm connected grid II'v of the firstaudio frequency amplifier and4A 40; varies. the, audio frequency voltage applied to` the grid of tubeV 4;

the winding or` relay1 IIJ;

II.A is adjustedto thecontactl I2 position, audio frequency howling takes. place. By adjusting switch I6 to the contact I3. position, reappearance; of the `carrier causes howling again and signies that thercarrier has. come on again.

It is tol be.` understood. that the relay device neednot, be operated; fromy the I. F; amplifier, or from; the second detector. It may be operated from. anyv other point` of the receiving system,

as long. as there isfavailable a variation in either current orvoltage due to the presence orabsence of areceiired signalfcar-rier; It is'tofbe further understood that aspeciallrectier could be em.- ployedzto operate the relay. Again, a less sensitive mechanical-or electronic relay may be employed. Inthat .case a directcurrent amplifier would be employed to increase the intensity of the variation control current or` voltage. The relaycouldalso, be arrangedtofcause an auxiliary audio frequency oscillatonorA other, audible signal source. to; feed intoA the audio frequency network ofV the receiver..

While I have indicated and described several systems; for.` carrying); my invention` into effect,- it will be apparent` to one skilled in, theart that'- my invention is by no: means: limited to thev particularorganizations. shown: and described, but that many' modifications maybe made without departing front the scope, of. my invention, as set forth in the appended claims'.

What I claim is:

l. In combination with a radio receiver of the type including. a'demodulator, a succeeding audio frequency network and 'a reproducer,` a carrier magnitude indication arrangement comprising a relay device responsive, to the presence or absence of the signal carrier, means operatively associated with said audio network for providing anzaudiofeedwork network in the receiver thereby to cause anzaudiblealarm signal to be emitted from said reproducer, and means responsive to said relay devicefor controlling said audio feedback.

2., In aradio receiver-comprising at yleast one carrier amplifier, ademodulator, an audio frequency amplifier and a reproducer; the improvement which comprises a mechanical relay device connectedy to the carrier amplifier `and arranged to be actuated by change in space current. of said carrier amplifier, means responsive to. carrier amplitude for controlling said space current, means providing an audio feedback circuit in saidy audio amplifier, and means responsive tosaid mechanical relay for completing said In the absenceof received carrier'A energy there is', no current" flow' throughA Hencethe armature. I I is in contact with contact point I2. If switch` through; thev direct. current, blockingI condenser; Eil: to`v theigrid. of` tube 4. Adjustment of. tap.

audiov feedback` circuit thereby'to cause anaudo..

howl tobe produced, by said reproducer.

3;.` In combination witha signalv carrier detector` deviceiprovidedwith a load impedance acrosswhich. is. developedv a voltage whose magnitudeisa direct function ofrcarrier magnitude, an-

audio frequency oscillation producing circuit means for reproducingk the audio oscillation, a

electronic. relay device whose impedance is function of said carrier-derived voltage, andsaid.

electronic impedance, being included in said audio oscillation. circuit thereby to control said audio oscillations.`

4. In combinationwith acarrer detector an an audio frequency network followed.v by a reproducer, means providing an audio feedback circuit in said audio network, a relay device having a circuit closing element thereof in said feedback circuit'. means responsive to rectified voltage developed byfsaid detector for-controlling the relay devicethereby tocontrol said element, and.

amplifier and adaptedto-be actuated bya change in space currentof saidf carrier amplifier, means responsive to carrier` amplitude forl controllingy said spacev current, means. providing an audio feedback circuit in said audio amplifier,A and means responsive to said relayfor completing.

vsaid audiofeedback circuit thereby to cause ann audio howl to beproduced by said reproducer.

6. In combination with a signal carrier detector device providedawith a load impedance across which 'is developed arectified voltage Whose magnitude. is a` functionv of carrier magnitude, an audiov frequency oscillation producing circuit,

means forireproducing the audio oscillations, an

JOHN R. SCHICK. 

